As shown in FIG. 1, in the conventional way an accelerometer sensor 8 of this kind includes an assembled head 9 forming the principal body of the sensor 8.
An assembled head 9 includes a barrel including a central bore 10 receiving a fixing element intended to fasten the accelerometer sensor 8 against a support element, for example an internal combustion engine cylinder block in the case of a knocking sensor. It is known to mold a plastic material coating 11 around an external wall of the assembled head 9 partly coating the assembled head 9.
Moreover, it is known that the sensor 8 includes piezoelectric members, one end of each member supporting an inertia mass. The members are accommodated in a cavity between the coating 11 and the assembled head 9.
In one embodiment of the piezoelectric members and their housing the cavity includes a support flange 13 formed by a portion of the external wall of the barrel projecting radially from the assembled head 9.
Said flange supports a piezoelectric member 12 surrounded by contact rings 3 serving as electrodes, a seismic mass 14 and a nut 15, all these components forming a stack.
At least one electrical connection pin 2 connected to each of the contact rings 3 extends out of the accelerometer sensor 8 via a protecting bush 16 itself molded into a portion of the coating 11 forming an arm.
Each connecting pin 2 is connected to a contact ring 3 by a strength lug 1, said strength lug 1 being electrically welded to the contact ring 3 and to the connecting pin 2. This is shown in FIG. 2 for a substantially cylindrical strength lug 1 interleaved between a connecting pin 2 and a contact ring 3 that are substantially plane, the contact ring 3 being the bottom element of these three elements.
The strength lugs 1 used in an accelerometer sensor of this kind have previously been made of copper, advantageously tinned copper. Copper is a technical facilitator of welding between the components of the contact ring 3 and the connecting pin 2, which consist of brass in the case of the contact ring 3 and bronze in the case of the connecting ring 2.
However, the disadvantage of using copper, advantageously tinned copper, as a component of a strength lug 1 is its lack of mechanical strength, notably the deterioration of its mechanical strength with a succession of heat cycles, the strength lugs being exposed to high temperature values.
It has therefore been proposed to replace copper with nickel for a strength lug 1. The position of the substantially cylindrical nickel strength lug 1 before welding is shown in FIG. 2.
This material is less suitable for welding, however. Electrically welding a nickel element such as a strength lug 1 of an accelerometer sensor produces flash, splashes 6 of molten material and wear of the electrodes. Nickel softens relatively little during welding. It also penetrates into the bronze of the other element, namely the connecting pin 2, forming a part 5 of the strength lug 1 melted into the connecting pin 2.
FIG. 3 shows the arrangement from FIG. 2 after welding. A portion of the strength lug 1 has melted into the electrical pin with too large a fused area 5. Similarly, the splashes 6 of molten nickel have reached the contact ring 3.
Nickel is electrically welded at a higher current than copper, for example toward 4 kA instead of 2.4 kA. The different melting points of the materials, namely nickel, bronze and brass, make the welding process difficult to control and also generate a final weld of poor quality. Thus the homogeneity of the weld is no longer guaranteed. Moreover, short-circuits can occur during the welding process.
For example, when the brass contact ring 3 is the bottom part of the three parts comprising the contact ring 3, the strength lug 1 and the electrical connection pin 2, the surface of the contact ring 3 facing the connecting pin 2 is held at a distance from the pin 2 by the strength lug 1 and receives splashes 6 of nickel from the strength lug 1. Moreover, the nickel of the strength lug 1 and consequently the strength lug 1 dig into the bronze of the electrical connection pin 2.
The problem at the base of the invention is to be able to weld reproducibly and efficiently a nickel strength lug with a bronze connecting pin and a brass contact ring in an accelerometer sensor, the strength lug being interleaved between the pin and the contact ring.